Question from a stright guy

I've always wondered...I've seen this so many times especially in a guy/guy relationship, when married one calls the other his "husband". How is that determined who is the "husband" and what is the other?? Do you flip a coin or something? How about in a female/female relationship, what is each called? Are the both the "Wife"? Not being critical, just don't know. Can somebody clear this up for me?